"Take it on the chin, you're a grown-up." People don't even have to speak the words for Craig Peters, 28, to know that's what they're thinking when he tells them his mum and dad are getting divorced. "If you're an adult when your parents split up, you're expected to take it in your stride, but I think it can be more damaging than when you're a child. You start to question all your childhood memories. You find your parents confiding in you and leaning on you in a way that they wouldn't have when you were a child. And I've had guilt to contend with too - my parents say they only stayed together all those years because of me."

Adult children of divorce, or Acods as they are increasingly known, are a fast-growing phenomenon. While the overall number of divorces has fallen for a third year in a row to its lowest level in 26 years, the number of over-60s choosing to end their marriages has increased by more than a third in the space of a decade.

Andrew Newbury, a partner at Pannone law firm, who is among those reporting a noticeable rise in couples parting after 40 or even 50 years together, says: "We had one pair choosing to separate aged 92."

Most of the so-called "silver splitters" that his firm has dealt with - more than 80 in the last year - were prompted by husbands taking up with other, often younger, women. "Empty-nest syndrome" and early retirement are also cited as triggers after people realise that the extra time they have together isn't as blissful as they'd hoped. People are wealthier than they used to be (even now) and more likely to be able to afford to divorce. Meanwhile, as the social stigma of divorce decreases, people are living longer and realise they may have a lot of years left.

Relate says that, these days, people of all ages are more demanding of relationships. "They're more demanding of life, too," says Newbury. "I repeatedly come across the feeling that the grass could be greener. It's like they're caught up in this '101 places you must visit' or 'books you must read before you die' mentality."

At least these parting couples can rest in the knowledge that the kids - who are almost inevitably adults - are OK. Or can they? What seems to be emerging is that the effect on adult children of divorce may be hugely underestimated.

For Craig, the sense of loss was overwhelming. "I've been surprised by how upset I've been, because at 28 you would assume you'd be past it and because I know that the divorce is the right thing for my parents. But it feels as though it's not just they who are separating, but us as a family. All that togetherness that I've taken for granted for nearly three decades has disappeared. It's very upsetting."

It's not as if people always see it coming. "I looked at my parents' marriage idealistically. They seemed to get on well and I had a great childhood," says Russell Hawkins, 26. "When they split up 18 months ago, it was as if my whole world suddenly had a big crack in it. I'm not saying it's easy, but if you're a child you adapt to things, whereas for 26 years I'd grown up with my parents' relationship as a constant and a rock. It's been a massive shock."

Russell adds: "A kid wouldn't usually have the emotional intelligence to work it [divorce] out and, in any case, your parents would probably try to protect you from it. When you're an adult, your parents confide in you, which makes it difficult not to take sides. Again, this adds to a sense of nothing being as you thought it was."

Paula Hall, a Relate counsellor and author of How to Have a Healthy Divorce, says that by being forced to question what they thought was ideal, or at least constant, many Acods start questioning all sorts of other things they've taken for granted, including their own relationships. "It's that sense of: 'Oh my goodness, is nothing permanent?' And: 'If my childhood wasn't what I thought it was, what else should I question?' We need more research into this unexamined group."

As it stands, the vast majority of funding for studying the effects of divorce is ploughed into examining outcomes for young children, although one study by the Joseph Rowntree Foundation recently included research into the life experiences of adults who were over 20 when their parents separated. It found that while women's economic circumstances were largely unaffected, men with late-divorcing parents tended to be socially and economically disadvantaged compared with peers with parents who stayed together. Meanwhile, men and women who were over 20 when their parents separated were more likely to have their own first partnership or marriage break up by the age of 33.

Noelle Fintushel, whose parents divorced when she was 22, was so dismayed at the lack of research that she sought out other Acods to investigate their feelings and experiences in the early 90s. When Nancy Hillard took an interest in her work, the two teamed up to compile information and personal stories from more than 100 adults whose parents had divorced when they were in their 20s and older.

Although their book, A Grief Out of Season, is now out of print, the fact that it is still highly sought after (some copies sell for more than £150) demonstrates Acods' need for support. "It is a big deal when parents divorce - no matter how old or independent their children," the book concludes. "Divorce shakes the roots of each [family] member's self-perception."

Fintushel remembers her own ideal family "blowing up in my face" and yet she had friends asking: "Why are you so upset? You're grown-up."

Rachel Cox, 32, says her parents' divorce was devastating. "They lost their 'deity status', which is quite destabilising and makes you feel quite alone." She noticed that her boyfriend and others whose parents were together seemed to still have them on a pedestal, "even if it's subconscious".

As she was growing up, Rachel says her parents never argued. "My friends would say to me: 'If your parents split up, I'd lose my faith in marriage.' But one day, when we were both grown-up, my sister discovered my mum was having an affair. She and I decided to give our mum a chance to tell our dad or we'd tell him ourselves, which is what we wound up doing. It got messy because she started trying to turn him against us, saying we were victimising her. When she opened up to us, she said the breakdown of the marriage wasn't about the affair, it was because she felt she'd had no real life, having given up a good job on marriage. Without me and my sister living at home, she started to feel more and more worthless."

Although Rachel now sympathises with her mother, at the time she felt angry. "I had my dad crying on one shoulder and my mum on the other. They'd say stuff about each other, too. Because I was grown-up, they appealed to my adult side. If I'd been a kid, no doubt they'd have tried to protect me. I found myself in massive shock, wondering how on earth I'd never noticed that my family was dysfunctional - and were all families that seemed happy dysfunctional? All my ideals were absolutely shattered. No matter how old you are, the child in you reacts."

Rachel says that as a direct result of her parents' break-up she split with her long-term partner and "for a while, I was crazily single - I mean, really determined to stay single." Eventually, she settled into a new relationship and started to rebuild the bridges with her mother. "But even now, there's still tension between us."

Victoria Anisman-Reiner, 25, also experienced tension years after her parents' divorce. "My father kept confiding in me and asking my advice, and though my mother never meant to put that burden on me, she let a lot more slip than she intended. I still feel torn at times between my loyalty and my love for each of them. It's the most difficult thing about this process."

Victoria regularly visits blogs and websites created by Acods: "It's invaluable being able to relate to someone else who's been there."

Lee Borden, a lawyer and divorce mediator, finds that older people who go through divorce are often so desperate for help, reassurance and validation that they lose all sense of appropriate boundaries. He recalls one of his clients and his wife coming into the office. "When I met them, there was a third person standing there - a young man in his mid-20s. 'This is my lawyer,' the wife said. 'Ted is our son,' my client said. I said a quiet prayer for both of them, and especially for Ted. His discomfort was obvious."

While this is an extreme case, Lee says that, all too often, he sees one spouse "moving quickly and shamelessly to line up allies among the adult children, telling them all the transgressions of the other parent throughout a lengthy marriage".

Angie Lensfield, who divorced in 2002, says that her son, then aged 22, has never forgiven either of his parents and estranged himself as a result. "He said the divorce ruined his life, that it was selfish of us to make him question everything solid in his life. It really surprised me and still hurts me because we were so close."

She believes that part of the reason was his anger at the sale of the "family" home. Indeed, many Acods report that, even if they never intended to use it, their parents' stable home was a touchstone they could always go to if they wanted.

The sudden role reversal hits some Acods hardest. "I rarely come across consensual divorce when people are over 50, with many cases involving one of them having an affair. The spouse who is left behind often leans heavily on their adult children," reports Marilyn Stowe of Stowe Family Law. "The balance tends to shift, and thus the fallout of an older divorce can shift disproportionately on to the children."

Often these parents depend on their children not only emotionally, but also practically. Laura Richards, 40, says: "My mum, who was in her late 60s at the time of the divorce, had never paid a bill. I had to show her how. She hadn't done any of this stuff before. Two years on, I still have to call her every day. In many ways, it was like my dad had died, only it was worse because it was his choice to leave her - and she spares me no details about her feelings towards him."

Laura also had to manage her children's anxieties about their grandparents' splitting up. "Explaining it was hard. My daughter said: 'Will you and Daddy get divorced, too, then?' We're this middle generation that has to deal with our own emotions and our children's."

It can come as an enormous relief to many Acods when their parents meet new partners, although the core feelings of protection, jealousy and fear of abandonment shouldn't be underestimated in adult children of divorce - particularly when the new partner has children that your parent may spend more time with than you.

Constance Ahrons, a sociologist and the author of We're Still Family: What Grown Children Have to Say About Their Parents' Divorce, says there is a further sticky area: money. "What I heard a lot in my interviews with people who were over 18 when their parents divorced were comments like 'My parents seem to be spending more money now they're separated,' or 'Together my parents seemed quite wealthy, but now the money pays for two homes and lifestyles, neither seems very well off at all,' or 'My dad has a new girlfriend and he's spending so much on her. If he marries her, will she get everything?' The divorced parents can become financially dependent on their adult children."

What most surprised Ahrons during her research was the number of Acods who were angry that their parents didn't divorce when they were younger. "I'm talking about the sons and daughters for whom the divorce didn't come as a shock. Many said: 'I'm the one that bore the brunt of this growing up. Why did you put me through it if you were going to do this in the end?'"

But there are positives. Laura says that she has become close to her father for the first time: "Before, I never saw my dad without my mum, but since the split, he and I have spent time talking and getting to know each other. That's been really nice."

Meanwhile, Craig says he is consciously working harder at his own relationship with his soon-to-be-wife "because I've seen what marriage can turn to. I would never want to go through a divorce."